Method and measuring device for locating enclosed objects

ABSTRACT

The invention relates to a method for locating objects enclosed in a medium, according to which a detection signal is generated by at least one capacitive sensor device. Said detection signal penetrates the medium that is to be analyzed in such a way that information is obtained about the objects that are enclosed in the medium by evaluating the detection signal, particularly by measuring impedance.  
     According to the invention, in order to obtain information about the depth of an object that is enclosed in the medium, a phase measurement of a variable which is correlated with a shift current of the capacitive sensor device is utilized.  
     The invention also relates to a measuring device for carrying out the inventive method.

BACKGROUND INFORMATION

[0001] The present invention relates to a method and/or a measuringdevice for locating objects enclosed in a medium according to thepreamble of Claim 1 and/or Claim 8.

[0002] A method of this nature, and/or a measuring device for carryingout this method utilizes a capacitive sensor device that generates adetection signal, e.g., in the form of an electromagnetic field, so thatthe detection signal passes through the medium to be analyzed, but, atthe very least, penetrates the medium to a sufficient extent. An objectenclosed in the medium influences the detection signal, so that anevaluation of the detection signal makes it possible to obtaininformation about an object that is enclosed in the medium.

[0003] A measuring device according to the general class, e.g., a studsensor, detects an object that is enclosed in the medium by way of thechange of the electrical capacitance of its capacitive sensor device,the change being generated by the enclosed object. An object that isenclosed in a medium changes the dielectric properties of the medium, sothat a precision capacitor that is brought into the vicinity of theobject senses a change in capacitance caused by the object and/or achange in its impedance. This capacitance change may be measured, forexample, by the shift current of the precision capacitor of thecapacitive sensor device.

[0004] A compact, hand-held stud sensor is made known in U.S. Pat. No.6,249,113 B1. To locate objects behind a surface, the stud sensormeasures the change in capacitance sensed by a sensor circuitry as themeasuring device is moved across a wall. To display the exact locationof an object enclosed in the medium, the measuring device according toU.S. Pat. No. 6,249,113 B1 comprises an LED array in an arrow-shapedformat on the housing of the measuring device. When an object isdetected by the measuring device, a pair of LEDs in the arrow-shaped LEDarray on the housing of the measuring device is activated as a functionof the signal strength. As the sensor is scanned closer to the enclosedobject, i.e., the stronger the detection signal that is generated by theobject becomes, the further the activated LEDs travel toward the arrowtip in the LED array. When the measuring device is finally positioneddirectly over the enclosed object, the tip of the arrow in the LED arrayis illuminated. Basically, therefore, the measuring device according toU.S. Pat. No. 6,249,113 B1 makes it possible to locate objects enclosedin a medium, e.g., a wall. Neither the device disclosed in U.S. Pat. No.6,249,113 B1 for locating objects enclosed in a medium, nor the verysimple method on which it is based, are capable of measuring the depthat which the object is located.

[0005] Publication WO 94/04932 discloses a portable device for locatingobjects positioned behind a surface, comprising a sensor for sensingadditional capacitive loading caused by the object, an evaluation unitfor the detection signal, and a display for presenting the measuredresults. In addition, the measuring device according to WO 94/04932comprises a device that allows the sensor device to be operated in ahigher-sensitivity or lower-sensitivity mode.

[0006] Publication WO 94/04932 further discloses a method fordetermining the location of an object positioned behind a surface. Toaccomplish this, the corresponding measuring device is moved across thewall to be analyzed. The sensor according to WO 94/04932 is capable ofsensing an increase or decrease in the thickness of the material. Thispermits the device to inform the operator, for example, that the sensorwas calibrated incorrectly, e.g., directly over an enclosed object. Themethod on which this is based further makes it possible to inform theoperator that the medium being analyzed is too thick or too thin for anenclosed object to be detected.

[0007] A digital register in the measuring device according to WO94/04932 permits the calibration data to be stored indefinitely whilethe sensor is powered on.

[0008] A stud sensor is made known in U.S. Pat. No. 6,198,271 B1, which,in order to locate objects enclosed in a wall, detects the changes incapacitance of three capacitive sensors as the sensor is moved acrossthe wall, the sensors being integrated in the measuring device. Acomparison circuit monitors the relative charge time associated witheach capacitive element, the charge times providing an indication of therelative capacitances of the three capacitive elements. Changes in therelative capacitances of the three elements as the device is moved alonga wall are due to a change in the dielectric constant of the wall, whichnormally results from the presence of an object behind the surface overwhich the device is moved. The comparison circuit uses differences inthe measured relative capacitances of the individual capacitive elementsto locate the enclosed object.

[0009] The measuring device disclosed in U.S. Pat. No. 6,198,271 B1includes a display that consists of a plurality of display elements thatare connected with the evaluation unit of the measuring device in such amanner that only those elements that are located directly above thelocated object display a signal. In this manner, it is possible tocenter the measuring device over the located object and therebyindirectly determine the location of the object.

ADVANTAGES OF THE INVENTION

[0010] The inventive method for locating objects enclosed in a mediumutilizes a detection signal that is generated by a capacitive sensordevice, the detection signal penetrating the medium to be analyzed andbeing influenced by an object located in the medium. By evaluating thedetection signal generated when an enclosed object is present incomparison with a detection signal that would be generated if an objectwere not present, information about objects enclosed in the medium maybe obtained. To detect the enclosed object, the inventive methodutilizes the change in capacitance of a capacitive sensor device thatresults from the change in the dielectric constants of the measuredmedium, the change being caused by the enclosed object. The change inthe dielectric constants may be determined by measuring the impedancebetween the electrodes of the capacitive sensor device.

[0011] According to the invention, the proposed method provides that, toobtain information about the depth of the object enclosed in the medium,a phase measurement is utilized, particularly a phase measurement of avariable which is correlated with a shift current of the capacitivesensor device.

[0012] If a voltage is applied between the electrodes of the capacitivesensor device, a stray electric field is produced that extends in anarea beyond the electrodes and can therefore engage in an object to beanalyzed. In advantageous fashion, a desired directional pattern mayalso be imposed on a field of this nature. If alternating voltage, inparticular, is applied between the electrodes of the sensor device, ashift current flows between the electrodes, along the electrical fluxlines that connect the two electrodes. When the voltage is fixed, thsshift current increases as the impedance of the precision capacitordecreases and the greater its capacitance becomes. If an object islocated in the area of the flux lines, the impedance between the sensorelectrodes changes and, therefore, the shift current changes. If one nowmeasures not only the magnitude of shift current but its phase as well,it is possible to not only determine the location of an enclosed objecton the surface of the enclosing medium, but to also obtain informationabout the depth of the object behind the surface of the medium.

[0013] Using the inventive method it is also possible to make adistinction between objects that are dielectrically “denser” (e.g.,copper wire) than the enclosing medium, and objects having lowerdielectric constants ε than the enclosing medium (e.g., plastic pipe).In the first case, the shift currents are strengthened by the enclosedobject, and they become weaker in the second case. This thereforeresults in a phase position that is altered by 180°, thereby enablingunequivocal identification of objects in terms of the magnitude of theirdielectric constants.

[0014] Advantageous improvements and further developments of the methodindicated in Claim 1 are possible due to the measures listed in thefurther claims.

[0015] The change in shift current and/or the change in capacitance ofthe sensor device induced by the presence of an enclosed object may bemeasured using the most diverse types of electronic circuitry. In theinventive method, the shift current is advantageously not selected toevaluate the detection signal directly. Instead, a measurement parameterthat has a linear relationship with the shift current of the capacitivesensor device is advantageously selected. Due to the fact that ameasurement parameter M and not the shift current itself is measured, itis possible to calculate, based on the measured signal, interferences inthe measured signal that result, e.g., from crosstalk effects or phasedistortions due to the frequency characteristic of the evaluationcircuitry.

[0016] In principle, any electrical measurement parameter is suitable,which is linked in any form with the impedance of the sensor element.The change in the shift current, and/or the change in the capacitance ofthe sensor device may be measured using the most diverse types ofelectronic circuitry. For example, the natural frequency of anoscillating circuit may be measured, the oscillating circuit beingcomposed of the precision capacitor and a coil that is connected with itin series or in parallel. After excitation by a brief electrical pulse,an oscillating circuit of this type performs a damped oscillation at itsresonance frequency. The time-resolved measurement of this resonancetherefore enables deductions to be made about the shift currentsinvolved. In advantageous fashion, with the inventive method for thedetermination of changes in impedance induced by the presence of anenclosed object, an electrical voltage is measured within an evaluationcircuitry for the detection signal. The magnitude and phases of thesevoltage values may be determined with high accuracy in simple fashionusing a corresponding sampling circuit.

[0017] By using a detection signal that is composed of a plurality offrequencies, and/or by using a spectrally broad detection signal, thesignal-to-noise ratio of the measurement may be improved, in particular.Moreover, by performing measurement and evaluation at more than onefrequency, it is possible to distinguish the located, enclosed object interms of its metallic-nonmetallic properties. When a plurality ofmeasuring frequencies is used, it is also possible, as an alternative,to simultaneously detect a plurality of targets at different depths,even though their signals are superposed. If the measurement wereperformed solely at a single frequency in this case, the measuredresults would be adulterated. Moreover, a spectrally broad electricalpulse used as the excitation signal allows ambiguities in phasemeasurement to be avoided by comparing the measured results at variousfrequencies.

[0018] In order to also be able to detect the most minute changes incapacitance caused by small, enclosed objects and/or objects whosedielectric constants differ unsubstantially from the constants of thesurrounding material, the inventive method provides that measuringfrequencies be utilized that are typically in an interval of 100 MHz to10,000 MHz. Advantageously, measuring frequencies are preferablyutilized for the detection signal that are located in an intervalbetween 1000 MHz and 5000 MHz. Optimal measuring frequencies for theinventive method and/or a measuring device that utilizes the method formfrequencies that are typically in an interval between 1500 MHz and 3500MHz.

[0019] The high frequencies themselves enable sufficiently large changesin the shift current in the presence of very small changes incapacitance caused by an enclosed object, so that such changes incapacitance and the objects that generate them can be measured with acorrespondingly high level of sensitivity. On the other hand, highfrequencies of this nature require measurement technology that iscorrespondingly complex. While phase displacement between current andvoltage on the capacitive sensor element is 90° at low frequencies,deviations from this are observed at higher frequencies, due toinductive effects. In addition to the observed changes in the imaginarypart of the impedance, significant ohmic portions in the impedance canalso be observed at higher frequencies, depending on the damping of thedielectric material of the precision capacitor. The inductiveinterference effects may be eliminated in the case of the inventivemethod by not measuring the dielectric shift current of the capacitivesensor device directly, but by measuring a measurement parametercorrelated in linear fashion with the shift current. Thefrequency-dependent constants, which establish the ratio of thedielectric shift current to the measurement parameter M measured in theinventive method, may be measured independently, resulting in twodegrees of freedom for compensating for interference effects. Thecoefficients can therefore be determined by performing a referencemeasurement on defined impedances and making them available to theinventive method during evaluation.

[0020] In the case of the inventive method, the detection signal forlocating an object enclosed in a medium is advantageously evaluated as afunction of the lateral displacement of the capacitive sensordevice—that generates the detection signal—on the surface of theenclosing medium. In this manner, it is possible to very accuratelymeasure the location of the enclosed object, i.e., its lateral positionin the enclosing medium, and a measurement of the sensor signals as afunction of the lateral displacement of the sensor over the objectallows the measurement accuracy to be increased further. Depending onthe displacement of the sensor, another ensemble of flux lines of theelectrical measurement field of the capacitive sensor device isinfluenced by the object. A characteristic dependence of the depth ofthe object and the lateral displacement on the phase position of themeasured signal therefore results.

[0021] The spatially resolved measurement of the detection signal canalso be utilized to better discriminate the background signal, which isgenerated solely by the enclosing medium.

[0022] Using a threshold sensor, which can also depend on the materialof the enclosing medium, for example, a decision step may beadvantageously implemented in the inventive method that determineswhether an enclosed object is present or not.

[0023] Likewise, a measure of the size of the object may be determinedvia the drop in signal strength based on the movement of the capacitivesensor device, which is advantageously coupled with a path sensor, sothat, with the inventive method, it is possible to determine the lateralposition of the object in the medium, its depth relative to the surfaceof the medium, and indications of the size of the object.

[0024] The inventive method may be used in advantageous fashion for acapacitive sensor device of a measuring device that serves to locateobjects enclosed in media. It is possible, in particular, using themethod according to the invention, to obtain a compact, hand-heldlocating device that permits the detection of objects enclosed in walls,ceilings and/or floors, for example, with a high level of accuracy.

[0025] In addition to a corresponding capacitive sensor device and themeans for generating and evaluating a detection signal of this sensordevice, an inventive measuring device of this nature also advantageouslyincludes an output device, e.g., a display, that permits the determinedmeasured results, in particular the location and depth of an objectenclosed in the medium, to be depicted in a spatially-resolved manner onthe display of the measuring device. To this end, a measuring device ofthis nature is equipped with a control and evaluation unit that isconnected with the sensor device and includes means that advantageouslyenable the spatially-resolved measured results to be depicted on thedisplay of the measuring device directly in real time, i.e., while themeasuring device is being moved across a wall.

[0026] The measuring device according to the invention and/or theinventive method on which it is based enable the operator to determinethe exact location of an object enclosed in a medium, in all threedimensions of space. It is also possible with the inventive method toobtain information about the size of the enclosed object.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] An exemplary embodiment of the inventive method is presented inthe drawing, and it is explained in greater detail in the descriptionhereinbelow. The figures in the drawing, their description, and theclaims directed to the inventive method and/or the measuring device thatutilizes the method contain numerous features in combination. Oneskilled in the art will advantageously consider them individually aswell and combine them into reasonable further combinations.

[0028]FIG. 1 is a schematic representation of the measurement situationon which the inventive method is based,

[0029]FIG. 2 is a block diagram for measuring impedance according to theinventive method,

[0030]FIG. 3 is a symbolic representation of the temperature dependanceof the evaluated measured signal M(ω),

[0031]FIG. 4 is a block diagram for depicting method steps for recordingreference values,

[0032]FIG. 5 is a block diagram for depicting the method steps in theinventive method.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033]FIG. 1 is a schematic representation of a typical measurementsituation for application of the inventive method and/or utilization ofthe measuring device. The objective is to detect an object 12 that isenclosed in a medium 10 using a capacitive sensor device 14. Enclosedobject 12 is located at a distance d from a surface 16 of enclosingmedium 10. A measuring device 18, which contains capacitive sensor 14,among other things, is placed on surface 16 of medium 10 that enclosesobject 12. Capacitive sensor device 14 is essentially composed of aprecision capacitor 20, which includes two capacitor electrodes 22 and24. Capacitor electrodes 22 and 24 are positioned side-by-side in FIG. 1merely to graphically illustrate the measurement principle. In a realcapacitive sensor device, the electrodes of a measurement capacitor willbe positioned essentially parallel to each other. The desireddirectional effect of the electric field of measurement capacitor 20 isgenerated by corresponding electrodes or geometric means.

[0034] By applying an electrical voltage 26, an electric field 28 isgenerated between electrodes 22 and 24 of measurement capacitor 20 ofmeasuring device 18. If an alternating voltage, in particular, isapplied to the two electrodes of the measurement capacitor, a “shiftcurrent” flows along flux lines 30 that describe electric field 28. Whenvoltage U is fixed, shift current I is greater the lower the impedanceis, i.e., complex resistance Z of measurement capacitor 20. Shiftcurrent I can be measured directly using an ammeter 21, for example, orusing a measurement parameter M correlated with the shift current, suchas a voltage signal.

[0035] Impedance Z of measurement capacitor 20 is essentially determinedby the material located between capacitor electrodes 22 and 24. If ameasurement capacitor 20 of this type is now brought into the vicinityof an enclosed object 12, the composition of the material changes in thearea covered by electric field 28.

[0036] In particular, the presence of an enclosed object 12 results in achanged dielectric constant ε and, therefore, changed impedance Z incomparison with a medium 10 in which an enclosed object 12 is notpresent.

[0037] The change in dielectric constants induced by the presence ofenclosed object 12 and the associated change in impedance Z of themeasurement capacitor corresponds to changed capacitance C of themeasurement capacitor.

[0038] The increase in capacitance C of measurement capacitor 20 and/orthe resultant increase in shift current I between capacitor electrodesis depicted in FIG. 1 using an increased density of flux lines in theillustration of electric field 28 in the flux line illustration.

[0039] When a material having a greater dielectric constant ε than thecorresponding constants of surrounding medium 10 enters field area 28generated by capacitive sensor 14, the flux lines become denser. If anobject has lower dielectric constants than the surrounding material, theflux line density lowers in the area of the enclosed object.

[0040] The change in capacitance caused by the presence of an enclosedobject and/or the change in shift current in the capacitive sensor maybe measured and evaluated using various electronic circuits.

[0041] For example, the natural frequency of an oscillating circuit thatforms can be utilized by the measurement capacitor and at least one coilconnected with it in series or in parallel. After excitation by a briefelectrical pulse, an oscillating circuit of this type performs a dampedoscillation at its resonance frequency. A time-resolved measurement ofthe resonance therefore enables deductions to be made about thecapacitances involved and, therefore, the shift current.

[0042] As an alternative, the shift current may be measured directly bythe measurement capacitor when a constant alternating voltage having afixed frequency is applied.

[0043] In the inventive method, electrical shift current I of capacitivesensor device 14 is not measured directly. Instead, to evaluate thedetection signal, a frequency-dependent measurement parameter M ismeasured, which has a nearly linear relationship with the shift currentof the capacitive sensor device. With the inventive method, anelectrical voltage correlated with the shift current, in particular, ismeasured as measurement parameter M. That is, the following applies formeasurement parameter M that is utilized:

M=M(ω)=α(ω)+β(ω)*I(ω)

[0044] Complex measurement parameter M(ω) is evaluated in linearapproximation of shift current I(ω) of the measurement capacitor. Inthis process, α(ω) describes an internal crosstalk of the capacitorelectrodes, and β(ω) takes into account the frequency characteristic andphase distortions on the electrical lines inside the evaluationcircuitry and the matching network of the capacitive sensor device.

[0045] α(ω) and β(ω) are frequency-dependent constants that are capableof being measured independently. They can be determined very exactly byperforming a reference measurement of defined impedances, for example,so that, by measuring M, the shift current is also measured.

[0046]FIG. 2 shows an exemplary embodiment of an evaluation circuit thatcan be utilized within the framework of the inventive method. A pulsegenerator 34 controlled by a time base 32 generates a chronologicallyshort, spectrally broad voltage pulse that can be supplied to capacitivesensor device 14 via a wave coupler 36. The capacitance of measurementcapacitor 20 and, therefore, impedance Z of the sensor are a function ofthe dielectric medium that penetrates the electric field of thecapacitor electrodes.

[0047] If the capacitive sensor device is brought into the vicinity ofan object 12, distortions of the electric field occur due to the changeddielectric constants of the capacitor field. Impedance Z is changed as aresult, and it is capable of being measured via the shift current and/orderived measurement parameter M(ω). The impedance of the capacitivesensor is coupled out once more as a time-dependent voltage signal U(t)by directional coupler 36, then it is amplified and forwarded to asampling unit 40, in which the magnitude and phase of the measuredsignal are determined. This will be described in greater detailhereinbelow.

[0048] At the point at which measurement sensor 14 is connected to thematch-terminated line, voltages coupled in by the generator via wavecoupler 36 are reflected in more or less pronounced fashion. Theamplitude and phase of the signal reflected at this point is areflection of the difference of impedance Z of sensor 14 and lineimpedance; it makes it possible to deduce the magnitude and phase of theimpedance of sensor 14 and, therefore to determine the magnitude andphase of the current flow through sensor 14.

[0049] The determination of the magnitude and phase of current flowthrough sensor capacitor 14 can therefore be traced back to thedetermination of magnitude and phase of voltage U reflected at theconnection point of sensor 14.

[0050] The signals reflected at the connection point pass back throughthe wave coupler. The signals induced in the transmitting branch viacrosstalk in wave coupler 36 are negligible in comparison with thesignal portions passing back directly in the direction of the detectingbranch. Voltage V that exists at the entry of the detection circuit is areflection of voltage U that is reflected at the connection point ofsensor 14, except for the minimal losses at wave coupler 36 and thetime-of-arrival difference.

[0051] The voltage (which is usually low) that results after wavecoupler 36 is advantageously amplified first in a high-frequencyamplifier 38 in the detecting branch. The voltage is then sampled atdefined points in time T. The points in time at which the voltage ismeasured is established by a sampling pulse. In order to enable adetermination of the phase of the reflected voltage relative to thephase of the voltage produced by the generator, it is important that thegenerator of the transmit signal and the generator of the sampling pulsebe coupled in a phase-locked manner. This is ensured by the use of thetime base.

[0052] The voltage portions that exist at sampler at frequency f, namely

V(f)=v(f)*exp(iφ(f))

[0053] therefore correlate with voltage W(T) measured after the sampleraccording to the equation

W(T)=Re(exp(I*2π*f*T)*V(f))

[0054] A shift in the point in time T at which sampling takes placetherefore allows the magnitude and phase of voltage V at frequency f tobe deduced.

[0055] Voltage W is advantageously processed first in a low-frequencyamplifier so it can then be detected in an analog-digital converter. Bymeasuring voltage W at various points in time T, it is thereforepossible to determine amplitude as well as the phase of the reflectedvoltage portions and thereby deduce the magnitude and phase of thecurrents flowing in the sensor.

[0056] The measured signal is forwarded to a digital signal processor 46in addition to analog-digital converter 44.

[0057] DSP element 46 performs further signal processing and control ofthe time base to generate the excitation pulse and the sampling pulse.DSP element 46 makes it possible for the evaluated measured values,i.e., the depth of objects enclosed in the wall, in particular, andtheir lateral position relative to the measurement sensor, to bedepicted in a display in real time, i.e., during the measurementprocedure itself. In this manner, it is possible with the inventivemethod to show the operator in a display where and at what depth in thewall objects are enclosed, even while the device is still being movedacross a wall, for instance.

[0058] To determine lateral position, the capacitive measuring devicecan be moved across the medium to be analyzed in two opposite directions50 and 52. A corresponding path sensor that forwards the currentposition of the capacitive sensor device to the digital signal processorpermits the correct depiction of the depth and lateral position of theobject.

[0059] For the inventive method, it is provided that, for calibrationpurposes, a defined reference impedance 54 can be measured instead ofmeasurement capacitor 20. To this end, the electrical circuit forgenerating and evaluating the detection signal has switching means forgenerating and evaluating the detection signal, the switching meansbeing depicted in the exemplary embodiment in FIG. 2 as symbolic switch56. The switching means permit the excitation pulse to be forwarded notto measurement capacitor 20, but to reference impedance 54 instead. Thisdefined reference impedance 54 can be generated, for example, byshort-circuiting the signal line. Another possibility for realizing adefined impedance inside the device is via an “open end” of the signalline, for example. In this manner, the inventive method and/or theinventive measuring device have a calibration device that is containedin the method and/or the device, which enables the method to compensatefor thermal drifts using mathematical means, for example.

[0060] It is therefore possible, in particular, by performing thecalibration measurement at defined impedance 54, to determine constantsα(ω) and β(ω), which are influenced by the electrical network andestablish the correlation between electrical shift current I of thecapacitive sensor device and measured parameter M(ω), and to compensatefor drifts of measured signal M(ω) that occur relative to shift currentI(ω) after a reference measurement of this type is carried out.

[0061] Substantial drift effects result primarily from temperaturechanges and aging processes in the components involved. For example,additional time delays δT can also occur between the excitation pulseand the interrogation pulse, which would result in distortions in thelow-frequency signal. Since an additional time delay of this nature onlyresults in a multiplicative factor in the case of theFourier-transformed measurement signal M(ω), a drift of the samplingpoint in time of this nature may be canceled in the data recordrelatively easily.

[0062] Moreover, the pulse power and spectral shape of the excitationpulse in particular can be subjected to thermal drift. A drift in thefrequency characteristic of the high-frequency amplifier may also becompensated for using a reference measurement of this nature.

[0063] To compensate for changes in the device, e.g.,temperature-induced drifts, a linear correction function for themeasured signal is used. FIG. 3 is a schematic representation of thetemperature influence on measurement parameter M(ω). Measurementparameter M(ω) is subject to strong temperature-dependent change. Curve56, for instance, shows the frequency-dependent measurement parameterM(ω) at a temperature of 20° C. Measurement curve 58, which is alsoshown, depicts signal M(ω) measured at a temperature of −10° C. Themethod used to evaluate the measured signal now assumes that there is alinear dependence between the two measurement curves at differenttemperatures.

[0064] To compensate for this temperature effect, two correction factorsγ0(ω) and γI(ω) [are used to establish]the correlation of measurementparameter M(ω) measured under calibration conditions (e.g., 20° C.) withmeasurement parameter M(ω) obtained in an on-site measurement. That is,the following applies, for example:

M ^(−10°)(ω)=γ0(ω)*(M^(20°)(ω))+γl(ω)

[0065] For the inventive method, for example, measurement parameter M(ω)is measured under calibration conditions, i.e., at a defined temperatureand reference impedance, which can be realized via air measurement, acalibration stone or a short-circuited sensor.

[0066] If a calibration measurement is now carried out on-site underreal operating conditions with the same defined impedance, i.e., an airmeasurement, a measurement of a calibration stone, or a measurement witha short-circuited sensor, correction constants γ0(ω) and γl(ω) can nowbe deduced from measured value M(ω), which has been changed due to drifteffects. The correction factors determined in this manner are stored ina memory unit, so they can be called up for subsequent signalevaluation.

[0067] If a calibration having defined impedance is carried out beforethe actual measurement to locate an enclosed object, correction factorsγ0(ω) and γl(ω) that are currently obtained in the calibrationmeasurement are also used to correct measurement parameter M(ω) in theactual measurement procedure.

[0068] In this manner, the inventive method is capable of cancelingeffects in the measured signal that have an adulterating influence onthe measurement parameter to be processed. These influences on themeasurement parameter of the capacitive sensor device, referred to ingeneral as drift effects, include, in particular, temperature changes,changes in humidity, changes caused by component aging, and changescaused by a variation in the voltage supplied to the measuring device.The exemplary embodiment of an inventive measuring device in the form ofa hand-held, battery-operated measuring device can therefore compensatefor a drop in battery voltage even over a certain period of time withoutthis variation in voltage having a marked influence on the quality ofthe measured results.

[0069] In addition to the drift effects described, sample strews ofindividual components also result in different measurementcharacteristics of each individual measuring device, which can becompensated for using the correction function described. The inventivemethod therefore enables compensation for drift effects or sample strewsby comparing a reference signal stored in the device with a calibrationsignal recorded at the point in time when the measurement is performed.This comparison measurement allows a linear correction parameter for themeasured signal to be determined, which permits the inventive method toback-calculate the values measured currently on-site to referenceconditions.

[0070] It is advantageous, in particular, to carry out a referencemeasurement directly after a device is fabricated, e.g., still in thefactory, under defined calibration conditions. This measurement can thenbe adjusted later based on the actual location measurements carried outon-site.

[0071] It is also possible to carry out a reference measurement of thistype that yields a defined measured signal M(ω) by using a “mastermeasuring device”, and to import the reference values determined for the“master device” in the form of a performance map into further measuringdevices immediately after production. In this case, it would also bepossible, for example, to compensate for sample strews of thedirectional pattern of the electric field of the precision capacitor ofthe individual devices.

[0072] Different directional patterns resulting from mechanical andgeometric differences in capacitor electrodes and/or correspondingdirectional electrodes for the electrical measurement field mean thereare differences in terms of the detected location of an enclosed object,and they make it difficult to compare the measured data obtained withthe various devices.

[0073]FIG. 4 uses a block diagram to illustrate the process of measuringreference values, which are measured in the factory directly after theinventive device was produced, for example, and which can be stored in amemory element of the measuring device. In step 90, user guidance iswritten to a memory element of the inventive measuring device, which iscapable of being reproduced on a display of the measuring device as ananimated film sequence, and thereby better inform the operator about themethod steps to carry out to calibrate the measuring device on-site.

[0074] In method step 92, reference measurements are carried out andstored in the device. The reference measurements serve to determine thesystem parameters specific to the device. To this end, the signalmeasured at defined impedances is evaluated, and a linear ordercorrection function is created for each individual measuring device.Using this correction function, it is possible to cancel sample strews,e.g., of the mechanical design of the capacitive sensor device, out ofthe signal measured later on-site.

[0075] In method step 92, the measuring device is calibrated againstvarious defined background materials. The values of these referencemeasurements, e.g., carried out on air, concrete, metal and Ytong stone,and further commonly used building materials, are stored in the device.By referencing the known dielectric constants of these definedmaterials, constants α(ω) and β(ω) may be determined; these constantsare conditional upon the detection network and represent therelationship between the dielectric shift current of the capacitivesensor device and measured signal M(ω) used for evaluation purposes. Areference measurement of this type also makes it possible to determinethe signal distortions that occur due to phase distortions and thefrequency characteristic of signal lines, and/or internal crosstalkbetween the electrodes of the precision capacitor. In this manner it ispossible to very accurately deduce the fundamental dielectric shiftcurrent when performing a subsequent determination of the measuredsignal M(ω) on-site by using coefficients α(ω) and β(ω), which are thenknown.

[0076] In method step 92, interpolation parameters are also obtained forthe model of the enclosing medium on which the method is based. Theinventive method uses a numeric model for the enclosing medium, whichutilizes a plurality of material parameters of defined referencematerials. By performing a reference optimization between the signal,measured on-site, from the surrounding medium with the parameters of themodel stored in the measuring device, it is possible to very exactlydetermine the dielectric properties of the surrounding medium that wasmeasured. Essentially, an interpolation of the reference parameters onwhich the model is based is utilized on the value of the enclosingmedium that is measured on-site.

[0077] In method step 96, a determination of a geometric factor of thecapacitive sensor device is carried out. To this end, a reference signalis measured on a defined, spatially very limited reference body that isenclosed in a known medium. Due to mechanical or geometric deviations ofthe electrodes of the capacitive sensor device, differences can occur inthe directional pattern of the precision capacitor, which would resultin uncertainty about the exact determination of the location of theenclosed object. In method step 96, correction parameters are thereforedetermined to take into account the deviations in the directionalpatterns of individual measuring devices for every individual measuringdevice, and they are stored in the measuring devices, so that theevaluating algorithm can call up these parameters and take them intoconsideration.

[0078] In method step 98 in FIG. 4, the factory setting of referencevalues for the inventive method and/or the inventive measuring devicedetermines threshold values for object detection based on the referencemeasurements that are carried out. Using these threshold values, theprocessing algorithm determines whether an object has been detected ornot. The threshold values are a function of the measurement accuracy ofevery individual device, and of corresponding sample strews.

[0079] Method step 100 in FIG. 4 represents the storage of predeterminedsettings in a memory element of the inventive measuring device. Usingthe stored reference values and a calibration measurement to be carriedout on-site before the actual measurement, it is possible to largelyeliminate interference effects on the measured signal, enabling anextremely accurate measurement sensor to be obtained. It should beemphasized, in particular, that plastic pipes can be detected using thismeasurement sensor as well, for example. Including a large number ofreference values that permit interference effects to be canceled duringsubsequent evaluation of the signal has a substantial effect on theenhanced performance of the inventive measuring device and/or theinventive method on which it is based.

[0080] A central point of the inventive method is to divide measuredsignal M(ω) to be evaluated into two parts. Measured signal M(ω) isdivided into a background part UG(ω), which originates from theenclosing medium, and an “enclosed” part E (ω), which results from theenclosed object.

[0081] The phase and amplitude of the “enclosed” signal and thebackground signal are known from the measurement of signal variableM(ω). It must be noted that “enclosed” signal E(ω), which originatesfrom enclosed dielectric objects, is extremely small. Changes incapacitance, which are determined when an enclosed object is present,typically take place in the sub-picofarad range when enclosed dielectricobjects, such as plastic pipes, are present. When an alternating voltageof, e.g., one volt, and having a measuring frequency of 100 KHz, isapplied to the capacitive sensor, the small changes therefore result indifferences in the shift current of less than one microampere.

[0082] For this reason, a measuring frequency in the gigahertz range isutilized with the inventive method in order to generate changes in themeasured signal that are sufficiently great, even when the smallestchanges in capacitance take place due to the presence of an enclosedobject. The background signal is the signal that would be generated ifobjects were not present. It can be measured directly next to anenclosed object, for example. The present invention takes advantage ofthe fact that the background signal is dominated by parts of the shiftcurrent that are generated by the areas of the electrical measurementfield that are close to the surface. From this point forward it isassumed that background signal UG(ω) is known. Background signal UG(ω)is composed of shift currents Iv(ω) along flux lines v of the electricfield of the precision capacitor. As shown in FIG. 1, for example, theindividual flux lines v are of different lengths. It is thereforepossible to define a mean flux line length L, which indicates the phaseof the shift current. From this point forward, all phases are indicatedin relation to this mean phase. If an enclosed dielectric object isbrought into the vicinity of the measuring electrodes of the capacitivesensor device, the current distribution of the shift current changes. Inpractice, one can assume that this change, which is caused by anenclosed object, is small. The following therefore applies:

E(ω)<<UG(ω).

[0083] It can therefore be approximately assumed that the influence ofthe dielectric field results in an amplification or attenuation of shiftcurrent Iv along individual flux lines v having length Lv. The followingtherefore applies:

Iv(ω)(in the presence of an enclosedobject)=ξ*Iv(ω)(background)*exp(i*2π/λ(ω)*(Lv−L))

[0084] In this case, ξ represents a real amplification or attenuationfactor. If the dielectric constant ε of the enclosed object is greaterthan the ε of the surrounding medium, then ξ>1. The capacitance of theprecision capacitor is increased, and the shift current is increased. Inthe opposite case, ξ<1. If the enclosed object is small enough, so thatonly flux lines having a certain length Lv are affected, then thefollowing approximately applies:E(ω) = lv(ω)(in  the  presence  of  an  enclosed  object) − lv(ω)(background))   = (1 − ξ)exp (i * 2π/λ(ω) * (L  v − L)) * l  v(ω)(background)

[0085] If the type of enclosed object is known, e.g., a metallicenclosed object or an open space, then the sign of (I−ξ) is known.

[0086] The following therefore applies:

2π/λ(ω)*(Lv−L)=−φ(ω)+ψ(ω)

[0087] That is, the length of the affected flux lines Lv may be deducedbased on a comparison of the phase of the signal E(ω) with the phase ofthe background signal UG(ω), based on the relationship:

λ(ω)/2π*(−φ(ω)+ψ(ω))+L=Lv

[0088] The length of the affected flux lines is related to the depth ofthe object via a geometric factor G(ω,L).

[0089] In practice, the device performs averaging over a locationinterval [x,y] in which largely no enclosed objects are present. Thespacial mean of MW_M(ω) therefore provides a usable starting point forthe background components. This means that, if the measurement parameterM(Xj,ω) was detected at n locations Xj, then, to determine thebackground components, all j greater than M(Xj,ω) are summed andnormalized with 1/N.

[0090] As a possible extension of this fundamental averaging method, itis advantageous to exclude areas with strong signal changes, i.e., largedeviations from the mean, from the averaging, or to replace thecalculation of the mean with the determination of the median of themeasured data obtained over the location.

[0091] Instead of performing an averaging of various locations, it isalso possible to utilize background signals MUG(ω) stored in a table inthe memory. If it is known, for example, that the background isconcrete, it is possible to utilize measured values MUGBETon(ω), whichresult for a homogeneous concrete block, that are stored in the memoryas the background signal. The stored background signal to be subtractedcan be selected automatically, e.g., by comparing an estimatedbackground signal with various backgrounds stored in a table, or via aswitch that the user can operate.

[0092] A numerical model is used for the background, the model utilizingat least four material parameters, e.g., the dielectric constants ofknown materials. The model is based on the reflectance behavior ofelectromagnetic signals on dielectric boundary layers. To determine thematerial of the enclosing medium that is measured, the weighting of theparameters in the model of the enclosing medium is varied until a modelsignal that comes as close to it as possible can be reconstructed byperforming a reference optimization in the measured background signal.The dielectric constants of the measured enclosing medium can thereforebe deduced based on an interpolation of the parameters of the modelmedium. When the dielectric constants of the enclosing medium are known,the depth of the enclosed object in the enclosing medium may be deducedfrom the phase information of the measured signal that originates fromthe enclosed object.

[0093] According to the invention, it is provided with the methoddescribed that the threshold for detecting enclosed objects is variable.A sensitivity setting allows, e.g., irrelevant objects, in particularthose having a periodic structure, to be canceled out of the measuredsignal, so they do not appear when the measured results are subsequentlydisplayed on an optical display. The inventive method permits themeasuring range to be limited to a desired depth range based on aselected special range of phase displacements of the measured signal. Inthis manner, the selection of a special, limited depth range may beimplemented. The measuring depth displayed in the optical display of theinventive measuring device may be switched between various values (e.g.,6 and 10 cm).

[0094]FIG. 5 is an overview of a block diagram to illustrate theindividual method steps in the inventive method.

[0095] After the device is powered on in step 60, a system query for themeasuring device takes place. System query 62 checks the battery status(battery voltage), for example, the internal resistance of the battery,and the current temperature. In step 64, a reference measurement at adefined impedance is carried out. To this end, a reference device insidethe device can be utilized, for example, or an air measurement can becarried out. This reference measurement is also carried out to determineEMC interferences, e.g., caused by adjacent transmitting equipment. EMCinterferences of this nature may be subsequently canceled in themeasured signal with the inventive method.

[0096] A wall contact check takes place in step 65 of the inventivemethod, in which the corresponding displacement transducer of theinventive measuring device performs a query to ensure that the measuringdevice is placed properly on the wall to be analyzed. As an alternative,the wall contact can also be queried by evaluating the measured signalof the capacitive sensor device. If the measuring device determines thatthe surrounding medium is air, then the device cannot be placed on thewall.

[0097] The actual measuring procedure then takes place in method step68, in which raw data from the capacitive sensor device is measured andforwarded to the digital signal processor. In method step 70, whichrepresents the start of the evaluation of the measured signal,interference signals from external sources of interference are canceledout of the raw data. In method step 72, a first correction of themeasured signal due to sample strews takes place. To accomplish this,the device-specific system parameters determined by a referencemeasurement performed in the factory, i.e., the corresponding correctioncoefficients, are taken into account and the measured signal istransformed in a described, linear fashion. Method step 74 describes thecorrection of drift effects internal to the device, such as temperatureand aging influences. To determine a corresponding correction functionfor measured signal M(ω), a comparison is carried out in method step 74between a reference measurement of a defined impedance carried out inthe factory and stored in the device, and the result of the actualreference measurement according to method step 64. For measured signalM*(ω) that has been processed in this manner, the described separationinto signal parts arising from the enclosing medium and signal partsthat originate from the enclosed object is now carried out in methodstep 76. The measured wall material is determined via interpolation withthe reference values using the parameters stored in the device forreference materials and a corresponding mathematical model for thecomposition of the enclosing medium. In particular, a dielectricconstant is assigned to the measured wall material and/or the enclosingmedium, which is required for the further evaluation of the measuredsignal.

[0098] After the detection signal is separated into signal partsoriginating from an enclosing medium and/or an enclosed object, ageometric factor for the capacitive sensor device is taken into accountin method step 78 to determine the exact location position of theenclosed object. This geometric factor takes into accountproduction-induced geometric deviations in the directional pattern ofthe capacitive sensor device, for example. These device-specificdifferences can be taken into account using a linear correction functionand canceled out of the actual measured signal. In order to take intoaccount the threshold values for object detection set at the factory,the decision is made via signal processing in method step 80 whether anobject has been located or not. If the decision is positive, the size ofthe object, its length relative to the measuring device, and the depthof the object are then determined via the described evaluation of themagnitude and phase of the measurement parameter M*(ω). In particular,the depth of the enclosed object in the wall is determined from thephase of the measurement parameter M*(ω) and the dielectric constant ofthe round material determined in method step 76.

[0099] In method step 82, the measured result that is obtained isdisplayed graphically on the display of the measuring device. Toaccomplish this, the position of the located object relative to thecurrent position of the measuring device, the object size and the depthof the object are depicted on the display device of the measuring deviceusing symbols in such a manner that the operator is provided with across-sectional representation of the analyzed wall.

[0100] In particular, it is also possible to display, graphically aswell, for example, a permissible drilling depth that is possible withoutcontacting the located object during drilling. The depiction of themeasured results on the display of the measuring device takes place inreal time, so that the located object is depicted on the display of theinventive measuring device with a minum time delay, even while themeasuring device is still being moved across a section of the wall.

[0101] The inventive method and the corresponding inventive measuringdevice are not limited to the exemplary embodiment presented in thedescription and the drawing.

What is claimed is:
 1. A method for locating objects enclosed in amedium, according to which a detection signal is generated by at leastone capacitive sensor device, the detection signal penetrating themedium that is to be analyzed in such a way that information is obtainedabout the objects that are enclosed in the medium by evaluating thedetection signal, particularly by measuring impedance, wherein, in orderto obtain information about the depth of an object that is enclosed inthe medium, a phase measurement of a variable which is correlated with ashift current of the capacitive sensor device is utilized.
 2. The methodas recited in claim 1, wherein a linear correlation between themeasurement parameter and the shift current of the capacitive sensordevice is utilized.
 3. The method as recited in claim 1, wherein anelectrical voltage signal is measured to evaluate the detection signalin terms of magnitude and phase.
 4. The method as recited in claim 1,wherein a phase shift of the capacitive sensor shift current generatedby an enclosed object is utilized to detect the object.
 5. The method asrecited in claim 1, wherein the detection signal is composed of morethan one measuring frequency.
 6. The method as recited in claim 5,wherein one or more measuring frequencies of the detection signal areused in an interval of 100 MHz to 10000 MHz, preferably in an intervalof 500 MHz to 5000 MHz, and, optimally, in an interval of 1000 MHz to3000 MHz.
 7. The method as recited in claim 1, wherein the detectionsignal for locating an object enclosed in a medium is measured andevaluated as a function of a lateral movement of a capacitive sensordevice that is generating the detection signal.
 8. A measuring device,in particular a hand-held locating device for locating objects enclosedin a medium, having a sensor device, with means for generating adetection signal for the sensor device, a control and evaluation unitfor determining measured values from the detection signal, and an outputdevice for the determined measuring devices, for carrying out a methodaccording to claim
 1. 9. The measuring device as recited in claim 8,wherein the measuring device includes means that permit measuredresults, in particular the location and/or the location and depth of anobject enclosed in a medium, to be depicted in a spatially resolvedmanner on a display device of the measuring device.